1. Field of the Invention
The present invention relates to a flow rate detection mechanism for mass flow meters and more particularly to an improved design and mounting a restriction element.
2. Description of Related Art
Flow rate detection mechanism of mass flow meters have been constructed in such a manner as to obtain a laminar flow pattern in a gas passage, and for this type of configuration, an etching plate is used or a plurality of capillary tubes are used at a bypass section, as described in Japanese Patent Publication No. Sho 59-41126.
As shown in FIG. 9, a flow rate detection mechanism is described in Japanese Patent Publication No. Hei 6-78926 and introduces gas from a passage 1P for detecting the gas flow rate and allows gas to flow between a flow rate throttle valve 2P and a tapered inner circumferential surface 4P of a hole formed on a housing 3P in order to obtain laminar gas flow. An inflow port 5P, provided at the tapered inner circumferential surface 4P which forms a laminar flow, diverts a part of the gas into a sensor tube, and ejects the gas at an outflow port 6P. The gas is distributed to a flow rate control section via the circumferential surface 4P downstream of the outflow port 5P and before the passage 7P. On the other hand, the remainder of gas is distributed to the flow rate control section via the circumferential surface 4P and passage 7P.
The flow rate detection mechanism of the mass flow controller described in U.S. Pat. No. 5,099,881 obtains laminar gas flow by allowing gas introduced from a passage 1P' to flow into an annular passage 4P' formed by a plug 2P' and a holder 3P' both having a tapered portion, as shown in FIG. 10. And from a through hole 5P', provided halfway in the annular passage 4P', part of the gas is diverted and introduced from an injection port 6P' to a sensor section 7P', and the flow rate of this gas is measured by the sensor section 7P', discharged from an extracting port 8P' to the downstream side of the passage 9P'. The remainder of gas is distributed to the flow rate control section via the annular passage 4P' and passage 9P'.
In the conventional flow rate detection mechanisms as described above, it is important to stabilize measurement values by forming a stable laminar flow in the tapered inner circumferential surface 4P or annular passage 4P' with a flow rate throttle valve 2P and plug 2P', to achieve uniform pressure distribution, and to divide the passage for measuring the flow rate from a portion in which the stable laminar flow is formed. Such mechanism require high accuracy in manufacturing.
For example, in FIG. 9, the flow rate throttle valve 2P is fastened with a screw 8P in order to obtain stable laminar flow by accurately aligning the center axis of the flow rate throttle valve 2P to the center axis of the passage 1P. However, this configuration has defects in that many components are required and they require high care to reduce the defects in processing, and also generate metallic powders by friction with the screw 8P when the flow rate throttle valve 2P rotates. This is a problem for measuring a gas flow rate when, in particular, gas that requires purity is allowed to pass as is the case of the gas used for semiconductor manufacturing processes.
In an example shown in FIG. 10, in order to align the center axis of plug 2P' to that of passage 1P', the basic profile of plug 2P' is ground with a lathe, and then the plug 2P' must be fixed to the passage 1P' by an extremely troublesome fabrication process. That is, with each of the conventional techniques described above, there are problems of high labor and costs in manufacturing a flow rate detection mechanism.
In addition, in the above-mentioned conventional techniques, the maximum flow rate of gas allowed to flow in passages 1P, 1P' must be restricted to a level that would not generate turbulence in the gas flow. That is, in the example of FIG. 9, since even a little disturbance that occurs in the pressure distribution of the gas flowing on the tapered inner circumferential surface 4P directly affects the measurement results, gas is only allowed to flow at a flow rate that would not cause turbulence in the gas flowing on the tapered inner circumferential surface 4P.
In the example shown in FIG. 10, disturbance occurring in the laminar gas flow on the upstream side of the portion with the through hole 5P' is able to be absorbed at an annular chamber 12P' located on the outer circumference of the holder 3P' to some extent, but any turbulence in the outer circumferential portion 13P' of the plug 2P' in free communication with the extracting port 8P' has serious effects on measurement results.
In the flow rate detection mechanism, as in the case of FIG. 10, gas is not only restricted to flow at a flow rate that would not cause turbulence around plug 2P' on the downstream side 13P' of the annular passage 4P', but also the center axes of the passage 9P', holder 3P', and plug 2P' must be meticulously aligned in order to prevent any occurrence of turbulence at the outer circumferential portion 13P' of plug 2P'. In addition, because the chamber 12P', as shown in FIG. 10, is intended to absorb pressure fluctuations by its volumetric capacity, a gas collection portion 14P' is generated in a passage causing degradation of replacement characteristics of gas when gas is changed over.
In all of the above-mentioned examples, the profiles of the flow throttle valve 2P and plug 2P' that can form stable laminar flow are those that can suppress the maximum flow rate of gas allowed to flow in passages 1P, 1P' to a specified limit, and it is a practical limit to allow gas to flow at a flow rate of about 20 L/s at flow throttle valve 2P and plug 2P' about 25 mm long. Consequently, to allow gas to flow accurately at a greater flow rate, the precision of the flow rate detection mechanism must be increased and this not only increases the manufacturing cost, but also causes the overall profile of the mass flow controller to be outside the planned dimensions.